Fabrication of interlayer dielectrics with high quality interfaces for quantum computing devices

ABSTRACT

A method includes: providing a first wafer including a first substrate, a first insulator layer on the first substrate, and a first dielectric layer on the first insulator layer; providing a second wafer including a second substrate, a second insulator layer on the second substrate, and a second dielectric layer on the second insulator layer; forming a first superconductor layer on the first dielectric layer; forming a second superconductor layer on the second dielectric layer; joining a surface of the first superconductor layer to a surface of the second superconductor layer to form a wafer stack; and forming a third superconductor layer on exposed first surface of the first dielectric layer.

TECHNICAL FIELD

The present disclosure relates to fabrication of interlayer dielectricswith high quality interfaces for quantum computing devices.

BACKGROUND

Quantum computing is a relatively new computing method that takesadvantage of quantum effects, such as superposition of basis states andentanglement to perform certain computations more efficiently than aclassical digital computer. In contrast to a digital computer, whichstores and manipulates information in the form of bits (e.g., a “1” or“0”), quantum computing systems can manipulate information using qubits.A qubit can refer to a quantum device that enables the superposition ofmultiple states (e.g., data in both the “0” and “1” state) and/or to thesuperposition of data, itself, in the multiple states. In accordancewith conventional terminology, the superposition of a “0” and “1” statein a quantum system may be represented, e.g., as α|0>+β|0>. The “0” and“1” states of a digital computer are analogous to the |0> and |1> basisstates, respectively of a qubit. The value |α|² represents theprobability that a qubit is in |0> state, whereas the value |β|²represents the probability that a qubit is in the |1> basis state.

SUMMARY

In general, in some aspects, the subject matter of the presentdisclosure can be embodied in methods that include: providing a firstwafer including a first substrate, a first insulator layer on the firstsubstrate, and a first dielectric layer on the first insulator layer;providing a second wafer including a second substrate, a secondinsulator layer on the second substrate, and a second dielectric layeron the second insulator layer; forming a first superconductor layer onthe first dielectric layer; forming a second superconductor layer on thesecond dielectric layer; joining a surface of the first superconductorlayer to (e.g., directly to) a surface of the second superconductorlayer to form a wafer stack; and forming a third superconductor layer ona first surface of the first dielectric layer.

Implementations of the methods can include one or more of the followingfeatures. For example, in some implementations, joining the surface ofthe first superconductor layer to the surface of the secondsuperconductor layer is performed under an ambient temperature less thanapproximately 150° C.

In some implementations, joining the surface of the first superconductorlayer to the surface of the second superconductor layer is performedunder an ambient temperature less than approximately 100° C.

In some implementations, joining the surface of the first superconductorlayer to the surface of the second superconductor layer is performedunder an ambient temperature between approximately 18° C. andapproximately 30° C., or more particularly between approximately 18° C.and approximately 26° C.

In some implementations, the methods further include ion milling thesurface of the first superconductor layer and the surface of the secondsuperconductor layer prior to joining the surface of the firstsuperconductor layer to the surface of the second superconductor layer.

In some implementations, joining the surface of the first superconductorlayer to the surface of the second superconductor layer includes bondingthe surface of the first superconductor layer to the surface of thesecond superconductor layer. The joining can be performed under a vacuumin the range of between approximately 10⁻⁷ Torr to approximately 10⁻⁹Torr.

In some implementations, the methods include removing a portion of thefirst substrate by performing a dry etch of the first substrate.Performing the dry etch can include exposing the first substrate to aSF₆ or XeF₂ plasma, in which the first insulator layer serves as an etchstop for the dry etch.

In some implementations, the methods include removing a portion of thefirst substrate by performing a wet etch of the first substrate.Performing the wet etch can include exposing the first substrate to asolution comprising KOH, in which the first insulator layer serves as anetch stop for the wet etch.

In some implementations, the methods removing a portion of the firstinsulator layer from the wafer stack by performing a wet etch of thefirst insulator layer. Performing the wet etch of the first insulatorlayer can include exposing the first insulator layer to a solutioncomprising HF, in which the first dielectric layer serves as an etchstop for the wet etch.

In some implementations, the methods further include, prior to formingthe first superconductor layer on the first dielectric layer, exposing asecond surface of the first dielectric layer, opposite of the firstsurface of the first dielectric layer, to an O₂ plasma, and prior toforming the second superconductor layer on the second dielectric layer,exposing a first surface of the second dielectric layer to an O₂ plasma.

In some implementations, the methods further include removing a portionof the first substrate and the first insulator to expose the firstsurface of the first dielectric layer, and, prior to forming the thirdsuperconductor layer, applying an O₂ plasma to the exposed surface ofthe first dielectric layer.

In some implementations, the methods further include forming athrough-hole that extends from the third superconductor layer to thefirst superconductor layer, and forming a superconductor interconnect inthe through-hole that is in contact with the first superconductor layerand the third superconductor layer. Forming the through-hole can includeperforming a SF₆ etch of the first dielectric layer, in which the firstsuperconductor layer serves as an etch stop for the SF₆ etch. Formingthe superconductor interconnect in the through-hole can includedepositing a superconductor material by electron-beam deposition or byphysical vapor deposition.

In some implementations, each of the first wafer and the second waferincludes a silicon-on-insulator stack.

In some implementations, the first substrate includes a first singlecrystal silicon substrate, the first insulator layer includes a firstSiO₂ layer or a first sapphire layer, and the first dielectric layerincludes a first silicon layer, and the second substrate includes asecond single crystal silicon substrate, the second insulator layerincludes a second SiO₂ layer or a second sapphire layer, and the seconddielectric layer includes a second silicon layer. The first siliconlayer can be a first single crystal silicon layer, and the secondsilicon layer can be a second single crystal silicon layer.

In some implementations, the first superconductor layer includes a firstaluminum layer, the second superconductor layer includes a secondaluminum layer, and the third superconductor layer includes a thirdaluminum layer.

In some implementations, the third superconductor layer, the firstdielectric layer, and the first superconductor layer form a parallelplate capacitor.

In some implementations, the third superconductor layer forms at leastpart of a quantum device. The quantum device can include a qubit.

In general, in some aspects, the subject matter of the presentdisclosure can be embodied in quantum devices that include: a dielectriclayer having a first surface and a second surface opposite to the firstsurface; and a first superconductor layer in contact with the firstsurface of the dielectric, in which a loss tangent of an interfacebetween the first surface of the dielectric and the first superconductoris less than or equal to a bulk loss tangent of the dielectric layer.

One or more implementations, embodiments, and/or aspects of thedisclosure can have various advantages. For example, in some cases, thedevices and/or methods disclosed herein can allow, in certainimplementations, for greater connectivity among quantum devices andincreases in quality factors associated with quantum devices, such asresonator devices. In some cases, the devices and/or methods provide asubstantial reduction of crosstalk. In some cases, the devices and/ormethods provide a reduction in surface loss at one or more interfaces.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features and advantages willbe apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G are schematics illustrating an example process forfabricating interlayer dielectrics having interfaces associated withrelatively high quality factor Q.

FIG. 2 is a flow chart that illustrates an example process forfabricating interlayer dielectrics.

DETAILED DESCRIPTION

Quantum computing entails coherently processing quantum informationstored in the qubits of a quantum computer. In certain types of quantumcomputing processors, such as quantum annealers, the qubits of thequantum processor are coupled together in a controllable manner suchthat the quantum state of each qubit affects the corresponding quantumstates of the other qubits to which it is coupled. Depending on theprocessor design, the chosen architecture may limit the total number ofqubits available for coupling, and thus limit the ability of theprocessor to perform complex problems requiring large numbers of qubits.Furthermore, in certain quantum computing designs, the qubits may sufferenergy loss and decoherence due to interactions with two level systems.In quantum computers that use qubits formed from superconductormaterials, such two level systems are typically found on the surfacesand interfaces of the materials used to fabricate the qubits and othercomponents of the computers. To expand the number of qubits availablefor coupling in a solid state quantum processor, such as a quantumannealer having superconducting quantum devices, one approach is toconstruct the quantum devices within the processor using 3D integration.That is, instead of fabricating the qubits and other quantum devices ofthe processor within a single plane that extends along one and/or twodimensions (e.g., x- and/or y-directions), the quantum devices can alsobe formed in one or more layers extending into a third dimension (e.g.,z-direction). 3D integration also allows, in certain implementations,for greater connectivity and increases in quality factors associatedwith resonator devices. Moreover, 3D integration can, in some cases,provide a substantial reduction of crosstalk, since multiple layers ofground planes can be used to isolate drivelines and undesired couplingto other qubits, among other sources of cross-talk.

Fabrication of integrated quantum devices with superconductingcomponents typically involves depositing dielectrics and metal layersusing, e.g., sputtering, electron beam evaporation, chemical vapordeposition, and/or atomic layer deposition, among other techniques.Formation of the integrated quantum devices can also entail patterningthe deposited layers. However, superconducting circuits, such as qubits,inductance based photon detectors, and linear resonators rely on lowloss reactive microwave components to achieve a high quality factor Q.The deposited dielectrics, which may be amorphous or polycrystalline,are typically very lossy compared to single crystal substrates (in somecases by many orders of magnitude). Therefore, such depositeddielectrics are not suitable for high coherence (low decoherence)superconducting quantum circuits.

An alternative to depositing poor quality dielectric materials is tointroduce high quality single crystal dielectrics using wafer bonding.However, even single crystalline dielectrics can have high loss if careis not taken to ensure that the interfaces are of high quality as well.For example, assuming a simple parallel plate capacitor having a 10 μmthick single crystalline dielectric (bulk loss tangent of about 10⁻⁶ orless) between the plates and a lossy interface approximately 3 nm inthickness at both the top and bottom metal-dielectric interfaces, inwhich the loss tangent of the lossy layer is 2*10⁻³ (a reasonableapproximation for lossy dielectrics), then the lossy interface stores (6nm/10 μm=6*10⁻⁴) of the total energy and the overall quality factor Qfor the parallel plate is 1/((6*10⁻⁴)*(2*10⁻³))=830,000 for a microwaveapplied to the device in which the microwave has a frequency in therange of between approximately 4 GHz and approximately 8 GHz, such asbetween 4 GHz and approximately 6.5 GHz. This quality factor issubstantially below that achievable with planar resonators, byapproximately an order of magnitude, even if it is assumed there is zeroloss from the high quality dielectric.

A factor that can contribute to the lossy interfaces of the singlecrystal dielectrics is the use of high temperatures (e.g., aboveapproximately 150° C.) during processing, which can lead to, e.g., theformation of hillocks, voids, and/or diffusion of materials across theinterface between dielectric and superconductor layers. Such losses areespecially true in cases where Al is used as the superconductormaterial. Typical cleaning steps for surfaces of dielectrics use hightemperatures (e.g., above approximately 150° C.). Additionally, waferbonding may require high temperatures. When one or more layers ofsuperconductor materials have already been deposited, any subsequenthigh temperature processing steps, such as cleaning or bonding, willallow the aforementioned interactions (hillock/void formation anddiffusion) to take place. Although the losses from the interface can bereduced somewhat by making the single crystal dielectric thicker, theincrease in dielectric thickness can complicate filling through-holevias in the dielectric with interconnects (e.g., thicker dielectriclayers can result in poor or no interconnection between layers due tolimits of deposition techniques).

In general, in some aspects, the present disclosure encompasses methodsfor fabricating interlayer dielectrics, in which the interlayerdielectrics exhibit high quality/low loss interfaces suitable for usewith quantum devices. The methods include providing a first wafer thatincludes a superconductor layer formed on a single crystal dielectricand a second wafer that also includes a superconductor layer formed on asingle crystal dielectric and then joining (e.g., bonding) thesuperconductor layers of each wafer together at temperatures belowapproximately 150° C. By joining the superconductor layers attemperatures below approximately 150° C., the fabrication methods reducediffusion and the formation of hillocks and voids between the dielectricand superconductor layers that can otherwise lead to lossy interfaces.Furthermore, the methods disclosed herein, including the joining (e.g.,bonding) process and the pre-cleaning process performed prior tojoining, can be performed without heat being added. For example, joining(e.g., bonding) the superconductor layers can be performed at an ambienttemperature of the manufacturing facility in which the device isconstructed, such as at room temperature (e.g., between approximately18° C. and approximately 30° C.). By performing the pre-cleaning stepand/or joining step without heat being added, losses associated with theuse of heat can be reduced, resulting in a higher quality (e.g., havinga lower loss tangent and a higher Q) interface once the superconductorlayer is formed on the dielectric. Additionally, since wafers havinghigh quality single crystal dielectrics are joined together, thestructures resulting from the fabrication methods disclosed herein canbe used as part of 3D integrated quantum circuits that exhibit lowlosses and that permit a substantial increase in the number of quantumdevices (e.g., superconducting qubits) that can be used relative tointegrated quantum circuits having a 2D or planar design.

FIG. 2 is a flow chart that illustrates an example process 500 forfabricating interlayer dielectrics with interfaces having low loss(e.g., associated with high Q when formed as part of a resonator) forquantum computing devices. FIGS. 1A-1G are schematics that illustratethe fabrication process set forth in FIG. 2. The views shown in FIGS.1A-1G correspond to side views of the fabrication process, in whichlayers extend into and out of the page. The size of the layers shown inthe figures is not intended to be to scale. The process 500 is describedwith reference to FIGS. 1A-1G as appropriate.

In a first step of the process 500, a first wafer 100 and second wafer200 are provided (502). FIG. 1A is a schematic that illustrates anexample of the first wafer 100. Though not shown, the second wafer 200has the same structure as the wafer 100 shown in FIG. 1A. The firstwafer 100 includes a substrate 108, an insulator layer 106, and adielectric layer 104. As explained above, dielectric layers that aredeposited using standard deposition techniques typically have anamorphous or polycrystalline structure with high loss that is notsuitable for use with quantum processors. To avoid such losses, thedielectric layer 104 in this example has a single crystalline structure.For example, the dielectric layer 104 can be un-doped single crystallineSi. The single crystalline dielectric layer 104 may be provided as partof, e.g., a silicon-on-insulator (SOI) wafer. SOI wafers are preciselyengineered multilayer semiconductor/dielectric structures that offerreduced parasitic capacitance in the top silicon layer due to theisolation provided the insulation layer on which the top silicon layeris arranged. The insulator layer 106 in an SOI wafer is typically SiO₂or sapphire, whereas the substrate 108 is typically bulk single crystalsilicon. In another implementation, the wafer could include asapphire-on-insulator wafer, in which a Si wafer (serving as thesubstrate 108) having an oxidized surface (serving as the insulatorlayer 106) is bonded directly to a sapphire wafer, followed by thinningof the sapphire wafer to form the dielectric layer 104. The substrate108 also can serve as a handling layer during fabrication that is laterremoved once the device is finalized. The thickness of the dielectriclayer 104 can be in the range of, e.g., about 1 to about 100 microns.The thickness of the insulator layer 106 can be in the range of about,0.05 to about 10 microns. Other thicknesses for the dielectric layer andinsulator layer can be used as well. In general, the process stepsdescribed herein are performed at relatively low temperatures (e.g.,below approximately 150° C.) such as at an ambient temperature of themanufacturing facility in which the device is fabricated.

Typically, an as-received SOI wafer also includes a thin native oxidelayer 102 on the surface of the single crystalline dielectric layer.Before processing the wafer 100, the thin native oxide layer 102 isremoved and a cleaning is performed to prepare the surface of the singlecrystalline dielectric layer 104 and provide a high quality interface.For instance, the wafer 100 may be dipped in HF to remove the nativeoxide and rinsed with de-ionized (DI) water followed by drying withnitrogen. Before the HF dip, contaminants are removed from the wafersurface. This entails, for example, performing a solvent cleaning withsonication of the surface, followed by either a dry oxygen cleaningmethod, or a wet cleaning method. For dry oxygen cleaning, numeroustechniques can be used such as, e.g., down stream oxygen ashing, ozone,direct plasma cleaning. For wet cleaning, the wafer may be subjected toa piranha etch containing a mixture of sulfuric acid (H₂SO₄) andhydrogen peroxide. Following the HF dip, in some implementations, thesurface of the single crystalline dielectric layer 104 can again beexposed to an O₂ plasma. The second wafer 200 also is prepared byremoving a native thin film oxide and by cleaning as described withrespect to the first wafer 100.

Next, a superconductor layer is formed on the dielectric layer of eachof the first wafer and the second wafer (504). For instance, as shown inFIG. 1B, a layer of superconductor material 110 is deposited on thecleaned surface of the single crystalline dielectric layer 104. Thesuperconductor material 110 is a material that exhibits superconductingproperties at or below a superconducting critical temperature, such asaluminum (superconducting critical temperature of 1.2 Kelvin) or niobium(superconducting critical temperature of 9.3 Kelvin). Othersuperconductor materials may be used instead. A superconductor materialalso is deposited directly onto the cleaned surface of the singlecrystalline dielectric layer of the second wafer as well. The depositioncan be performed using, e.g., a physical vapor deposition process suchas sputtering, thermal evaporation, or e-beam deposition. Otherdeposition processes may be used instead. The thickness of the depositedsuperconductor layer 110 can be selected based on the desired end usefor the superconductor (e.g., whether the layer 110 will be part of aground plane, a plate of a capacitor, a routing wire, a microstrip, aresonator, or other component). As an example the thickness of thesuperconductor layer 100 can range between about 1 nm to about 1 micron,though other thicknesses are also possible.

After depositing the superconductor layer 110 on each of the dielectriclayers of the two wafers, the exposed surfaces of superconductor layers110 are prepared for joining (e.g., bonding). Preparation of the exposedsurfaces of the superconductor layers includes performing an ion milling(represented by dashed lines 112 in FIG. 1B) of the superconductorlayers 110. Ion milling includes directing ions (e.g., argon ions) atthe surface of the superconductor layer 110 at an angle and sputteringmaterial from the surface. For example, in some implementations, ionmilling can be performed in an ion milling machine by flowing Ar gas atabout 3.6 sccm at a pressure of 3.7*10⁻⁴ mBar, without heat being added(e.g., under an ambient temperature of a manufacturing facility in whichthe device is being fabricated). It is believed that the ion millingactivates the surface structure of the superconductor material toprovide improved bonding properties. That is, by removing the nativeoxide and/or contaminants from the surface of the superconductor usingan ion mill, a room temperature or near room temperature wafer bondingcan be achieved (e.g., at about an ambient temperature of themanufacturing facility in which the device is fabricated). Because theion milling process only removes atoms from the surface of thesuperconductor layer 110, however, the ion milling does notsubstantially increase the energy loss associated with the surface ofthe superconductor layer 110.

Following the ion mill activation of the superconductor layers 110 ofeach wafer, the first wafer 100 and the second wafer 200 are joined(e.g., bonded) together (506), as shown in the example of FIG. 1C toform a wafer stack. As explained above, the second wafer 200 is preparedin the same manner as the first wafer 100 and includes a substrate 208,an insulator layer 206, a single crystalline dielectric layer 204, and asuperconductor layer 210. Bonding is performed under a vacuum. The rangeof pressure for the vacuum can be between approximately 10⁻⁷ Torr toapproximately 10⁻⁹ Torr. Bonding may require relatively little pressure.For instance, a few pounds of pressure (for a wafer of approximately 3inch diameter, e.g., about a few kPa) may be sufficient to obtain anadequate bond between the wafers as in a cold-welding process. Forexample, to achieve adequate bonding of a first superconductor layer ofa first wafer to a second superconductor layer of a second wafer in awafer stack, such as the stack shown in FIG. 1C, the applied pressure(applied, e.g., in a direction orthogonal to the surfaces being bonded)can be a non-zero pressure that is less than or equal to 1 kPa, lessthan or equal to 2 kPa, less than or equal to 3 kPa, less than or equalto 4 kPa, less than or equal to 5 kPa, less than or equal to 6 kPa, lessthan or equal to 7 kPa, less than or equal to 8 kPa, or less than orequal to 9 kPa, among other pressures. In some implementations, however,greater pressure may be applied. The bonding also can be performed at anambient temperature of the manufacturing facility in which the device isconstructed (e.g., in the range of approximately 18° C. andapproximately 30° C.). An advantage of performing the bonding withoutadding heat, is that the formation of hillocks and voids, and thediffusion that would otherwise occur, are absent so that the interfacebetween the single crystalline dielectric layers and the superconductorlayers maintains their low loss properties. For example, in someimplementations, the loss tangent of the interface between the singlecrystalline dielectric layer and the superconductor layer can be equalto or less than the bulk loss tangent of the dielectric layer. Forinstance, the loss tangent of the interface can be less than or equal to10⁻⁶ (at a temperature of 50 mK and an applied field between 4-6 GHz).In addition, the absence of heat being applied during the bondingprocess also prevents an increase in loss that would occur at thebonding interface between the superconductor layers 110 and 210. In someimplementations, additional heat can be applied during the bondingprocess if desired. However, the temperature should preferably be keptbelow approximately 150° C., which is the typical temperature at whichthe hillocks and/or voids begin to form and diffusion across interfacesincreases sufficiently enough to generate noticeable losses at theinterfaces between the dielectric and superconductor and between thesuperconductors. For example, the bonding process can be performedbetween approximately 18° C. and approximately 150° C., such as betweenapproximately 100° C. and approximately 150° C., between approximately18° C. and approximately 100° C., or between approximately 18° C. andapproximately 30° C. An approximate temperature value can have ameasurement uncertainty within plus or minus 1 degree of the statedtemperature. The relatively low temperature bonding can be performedusing, e.g., the Bond Meister available from Mitsubishi HeavyIndustries, Ltd.

After bonding the two wafers together, the handling substrate (e.g.,substrate 208) of at least one of the wafers is removed (508) from thewafer stack 300 as shown in FIG. 1D. Though substrate 208 is shown beingremoved, substrate 108 can be removed instead or in addition tosubstrate 208. The handling substrate can be removed using a dry or wetetch technique. For instance, if the handling substrate is silicon, thesubstrate can be removed by applying a SF₆ plasma etch or a XeF₂ plasmaetch. Other dry etches are also possible. Alternatively, for a siliconsubstrate, the substrate can be removed by applying a KOH wet etch. Ifthe insulating layer 206 is SiO₂, then the insulating layer 206 canserve as an etch stop layer for either the dry or wet etch so that it isnot necessary to continuously check whether the etch process has beencompleted. The etch of the handling substrate 208 is preferablyperformed without adding heat to the etch (e.g., under an ambienttemperature of a manufacturing facility in which the device isfabricated) or at a relatively low temperature below 150° C.

Subsequent to removing the handling layer 208, the insulating layer thatwas located beneath the removed substrate then is removed (510) as shownin the example of FIG. 1E. The insulating layer may be removed usingeither a dry etch or wet etch. For example, if the insulating layer isSiO₂, a wet etch such as an HF dip can be used to remove the insulatinglayer 206. As with the removal of the handling substrate 208, theinsulating layer 206 is preferably etched without adding heat (e.g.under an ambient temperature of the manufacturing facility in which thedevice is being fabricated) or at a relatively low temperature belowapproximately 150° C.

After the insulating layer 206 is removed, the now exposed surface ofthe single crystalline dielectric 204 is cleaned in preparation fordeposition of another superconductor layer. For instance, the cleaningstep can include applying an O₂ plasma to the surface of the dielectriclayer 204. Alternatively, or in addition, the cleaning step can includeremoving oxides using a fluorine chemistry on the surface, so that thesurface can be made selective to Al and Si substrate. For instance, thefluorine chemistry can include applying a HF acid dip, exposing thesurface to HF vapor, or XeF₂ gas, among others Subsequently, anothersuperconductor layer 302 is deposited on the cleaned exposed surface ofthe dielectric layer 204 (512) as shown in the example of FIG. 1F. Thesuperconductor material can include, e.g., aluminum or niobium, thoughother superconductor materials are also possible. The superconductorlayer 302 can be deposited using a physical vapor deposition techniquesuch as thermal evaporation, sputtering, e-beam deposition. Otherdeposition techniques may be used instead. The thickness of thedeposited superconductor layer 302 can be selected based on the desiredend use for the superconductor (e.g., whether the layer 302 will be partof a ground plane, a plate of a capacitor, a routing wire, a microstrip,a resonator, or other component). As an example the thickness of thesuperconductor layer 302 can range between about 1 nm to about 1 micron,though other thicknesses are also possible. The deposition of thesuperconductor layer 302 is preferably performed without heat beingadded (e.g., at an ambient temperature of the manufacturing facility inwhich the device is formed) or at a relatively low temperature belowapproximately 150° C. (e.g., a thermal evaporation process may result inresidual heat being added to the device as the superconductor isdeposited). In some implementations, the substrate 108, insulator layer106, and/or dielectric layer 104 can be removed depending on devicedesired. For example, the final device may just include layers ofsuperconductor and interlayer dielectrics to form one or more stackedcapacitor devices. Alternatively, in some implementations, the substrate108 can be retained as a handling wafer if, e.g., further processing isdesired.

The process thus described with reference to FIGS. 1A-1F is sufficientto form a simple parallel plate capacitor having a relatively highquality factor Q, in which the last formed superconductor layer 302 isone plate of the capacitor, the silicon layer 204 is the dielectric, andthe superconductor layers 110, 210 are the other plate of the capacitor.In some implementations, one or more of the superconductor layers andthe dielectric layers can be patterned to form other components that canbe used as a quantum device or in a quantum processor. For instance, theprocesses disclosed herein can be used to form some or all of groundplanes, microstrips, resonators, superconducting qubits (e.g., a fluxqubit such as a persistent current flux qubit), or routing wires. Theprocesses disclosed herein can be used to form other components ofintegrated quantum devices as well. Using the fabrication techniquesdisclosed herein, quantum devices can be produced having an overallquality factor Q, for an applied microwave electromagnetic field havinga frequency between approximately 4 GHz and approximately 8 GHz (e.g.,between approximately 4 GHz and approximately 6.5 GHz) at approximately50 mK that is greater than about 5*10⁶, such as, e.g., greater thanabout 1*10⁷, greater than about 5*10⁷, greater than about 1*10⁸, greaterthan about 5*10⁸, or greater than about 1*10⁹. In some implementations,forming such components may require additional patterning of the wafersbefore or after bonding. For example, in some implementations, thesuperconductor layers 110, 210 and dielectric layers 104, 204 of eachwafer may be patterned before bonding using lithography and etchingtechniques as known from classical integrated circuit manufacturing.Alternatively, or in addition, the superconductor layer 302 and/ordielectric layer 204 may be patterned before or after deposition of thesuperconductor layer 302.

For example, FIG. 1G is a schematic that shows the formation of athrough-hole 402 and an interconnect 404 that electrically connects thetop superconductor layer 302 to the bottom superconductor layer 210. Ifthe superconductor layer 302 has already been deposited on the surfaceof the dielectric 204, the through-hole 402 can be formed by removing aselected portion of layer 302 (e.g., using lithography and a metal etch)to create an opening in layer 302. Then the opening in layer 302 isexposed to, e.g., a dry etch such as SF₆ to etch the dielectric layer204 and form the through-hole 402. In this case, the superconductorlayer 210 can serve as an etch stop for the SF₆ etch. Followingformation of the through-hole 402, the superconductor material can bedeposited (e.g., using physical vapor deposition as disclosed herein) inthe through-hole 402 to form an interconnect 404. The interconnect 404can form on the sidewalls of the dielectric layer 204 and the surface oflayer 302.

In some implementations, the interconnect 404 fills the entirety of thethrough-hole 402. In some implementations, the dielectric layer 204 canbe patterned and etched first to form the through-hole 402. Thesuperconductor material then can be deposited on the surface of thedielectric layer 204 and in the through-hole 402 to form thesuperconductor layer 302 and the interconnect 404. However, this latterapproach would require forming, e.g., a photoresist directly on thedielectric layer 204 in order to define the through-hole area. Suchresist can be difficult to completely remove leading to higher losses atthe interface between the dielectric and the subsequently depositedsuperconductor material. An advantage of the techniques disclosed hereinis that they allow formation of through substrate vias (TSVs) usingsuperconductors in relatively thick materials. That is, when the aspectratio (depth to width) is relatively high, TSVs are difficult to form,especially with superconductors, since the openings do not fillproperly, leading to poor connectivity. However, by forming the TSV instages using the presently disclosed techniques, a more reliable TSV maybe established. For example, 50 μm thick dielectric layer can bepatterned to form a through-hole and filled with a superconductor toestablish a 50 μm TSV. Then, the process can be repeated with another 50μm thick dielectric layer forming a second 50 μm TSV. The two layers canbe arranged so the first and second TSV line up with one anotherfollowed by bonding of the layers together, such that the two separate50 μm TSVs form a single 100 μm TSV.

Additional layers and components can be formed and added by repeatingthe process steps described with respect to FIGS. 1A-1F one or moretimes. For instance, a third SOI wafer containing a patternedsuperconductor layer can be prepared, aligned and bonded to thesuperconductor layer 302 using the techniques as described herein toform additional capacitors, ground planes, wiring, and/or superconductorqubits, among other devices. Though the process set forth herein isdescribed with respect to the fabrication of components for quantumcomputing systems, the same processes can be used in fabrication ofclassical circuit components, e.g., CMOS devices, as well. Inparticular, the processes described herein can be useful for fabricatingsystems/components where high quality interlayer dielectrics having lowloss are desired.

Embodiments of the quantum subject matter and quantum operationsdescribed in this specification can be implemented in suitable quantumcircuitry or, more generally, quantum computational systems, includingthe structures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. The term“quantum computational systems” may include, but is not limited to,quantum computers, quantum information processing systems, quantumcryptography systems, or quantum simulators.

The terms quantum information and quantum data refer to information ordata that is carried by, held or stored in quantum systems, where thesmallest non-trivial system is a qubit, i.e., a system that defines theunit of quantum information. It is understood that the term “qubit”encompasses all quantum systems that may be suitably approximated as atwo-level system in the corresponding context. Such quantum systems mayinclude multi-level systems, e.g., with two or more levels. By way ofexample, such systems can include atoms, electrons, photons, ions orsuperconducting qubits. In many implementations the computational basisstates are identified with the ground and first excited states, howeverit is understood that other setups where the computational states areidentified with higher level excited states are possible. It isunderstood that quantum memories are devices that can store quantum datafor a long time with high fidelity and efficiency, e.g., light-matterinterfaces where light is used for transmission and matter for storingand preserving the quantum features of quantum data such assuperposition or quantum coherence.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments. Certain features that are described in thisspecification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable sub-combination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a sub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. For example, the actions recited in the claims can be performedin a different order and still achieve desirable results. In certaincircumstances, multitasking and parallel processing may be advantageous.Moreover, the separation of various components in the embodimentsdescribed above should not be understood as requiring such separation inall embodiments.

Particular embodiments of the subject matter have been described. Otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A method comprising: providing a first wafercomprising a first substrate, a first insulator layer on the firstsubstrate, and a first dielectric layer on the first insulator layer;providing a second wafer comprising a second substrate, a secondinsulator layer on the second substrate, and a second dielectric layeron the second insulator layer; forming a first superconductor layer onthe first dielectric layer; forming a second superconductor layer on thesecond dielectric layer; joining a surface of the first superconductorlayer to a surface of the second superconductor layer to form a waferstack; and forming a third superconductor layer on a first surface ofthe first dielectric layer.
 2. The method of claim 1, further comprisingion milling the surface of the first superconductor layer and thesurface of the second superconductor layer prior to joining the surfaceof the first superconductor layer to the surface of the secondsuperconductor layer.
 3. The method of claim 2, wherein joining thesurface of the first superconductor layer to the surface of the secondsuperconductor layer comprises bonding the surface of the firstsuperconductor layer to the surface of the second superconductor layer.4. The method of claim 3, wherein joining the surface of the firstsuperconductor layer to the surface of the second superconductor layeris performed under a vacuum in the range of between approximately 10⁻⁷Torr to approximately 10⁻⁹ Torr.
 5. The method of claim 2, comprisingremoving a portion of the first substrate from the wafer stack byperforming a dry etch of the first substrate.
 6. The method of claim 5,wherein performing the dry etch comprises exposing the first substrateto a SF₆ or XeF₂ plasma, and wherein the first insulator layer serves asan etch stop for the dry etch.
 7. The method of claim 2, comprisingremoving a portion of the first substrate from the wafer stack byperforming a wet etch of the first substrate.
 8. The method of claim 7,wherein performing the wet etch comprises exposing the first substrateto a solution comprising KOH, and wherein the first insulator layerserves as an etch stop for the wet etch.
 9. The method of claim 2,comprising removing a portion of the first insulator layer from thewafer stack by performing a wet etch of the first insulator layer. 10.The method of claim 9, wherein performing the wet etch of the firstinsulator layer comprises exposing the first insulator layer to asolution comprising HF, and wherein the first dielectric layer serves asan etch stop for the wet etch.
 11. The method of claim 2, furthercomprising: prior to forming the first superconductor layer on the firstdielectric layer, exposing a second surface of the first dielectriclayer, opposite of the first surface of the first dielectric layer, toan O₂ plasma; and prior to forming the second superconductor layer onthe second dielectric layer, exposing a first surface of the seconddielectric layer to an O₂ plasma.
 12. The method of claim 2, furthercomprising: removing a portion of the first substrate and the firstinsulator to expose the first surface of the first dielectric layer; andprior to forming the third superconductor layer, applying an O₂ plasmato the exposed first surface of the first dielectric layer.
 13. Themethod of claim 2, further comprising: forming a through-hole thatextends from the third superconductor layer to the first superconductorlayer; and forming a superconductor interconnect in the through-holethat is in contact with the first superconductor layer and the thirdsuperconductor layer.
 14. The method of claim 13, wherein forming thethrough-hole comprises performing a SF₆ etch of the first dielectriclayer, wherein the first superconductor layer serves as an etch stop forthe SF₆ etch.
 15. The method of claim 13, wherein forming thesuperconductor interconnect in the through-hole comprises depositing asuperconductor material by electron-beam deposition or by physical vapordeposition.
 16. The method of claim 2, wherein each of the first waferand the second wafer comprises a silicon-on-insulator stack.
 17. Themethod of claim 2, wherein the first substrate comprises a first singlecrystal silicon substrate, the first insulator layer comprises a firstSiO₂ layer or a first sapphire layer, and the first dielectric layercomprises a first silicon layer, and wherein the second substratecomprises a second single crystal silicon substrate, the secondinsulator layer comprises a second SiO₂ layer or a second sapphirelayer, and the second dielectric layer comprises a second silicon layer.18. The method of claim 17, wherein the first silicon layer is a firstsingle crystal silicon layer, and wherein the second silicon layer is asecond single crystal silicon layer.
 19. The method of claim 2, whereinthe first superconductor layer comprises a first aluminum layer, thesecond superconductor layer comprises a second aluminum layer, and thethird superconductor layer comprises a third aluminum layer.
 20. Themethod of claim 2, wherein the third superconductor layer, the firstdielectric layer, and the first superconductor layer form a parallelplate capacitor.
 21. The method of claim 2, wherein the thirdsuperconductor layer forms at least part of a quantum device.
 22. Themethod of claim 21, wherein the quantum device comprises a qubit.
 23. Aquantum device comprising: a dielectric layer having a first surface anda second surface opposite to the first surface; and a firstsuperconductor layer in contact with the first surface of the dielectriclayer; wherein a loss tangent of an interface between the first surfaceof the dielectric layer and the first superconductor is equal to or lessthan a bulk loss tangent of the dielectric layer.